1. Field of the Invention
The present invention relates to a vehicle surroundings monitoring apparatus that monitors vehicle surroundings on the basis of distance data calculated from a pair of stereo images.
2. Description of the Related Art
In recent years, stereo-type vehicle surroundings monitoring apparatuses, which use a pair of vehicle-mounted cameras (stereo cameras) each incorporating a solid-state image pickup element such as a CCD, have attracted attention. In order to recognize vehicle surroundings with these vehicle surroundings monitoring apparatuses, first, a pixel block having correlation with each pixel block in one image is specified in the other image (stereo matching), and a parallax is calculated as a relative displacement amount between the pixel blocks. Distance data is calculated from parallaxes, which are obtained for pixel blocks, according to the principle of triangulation. By subjecting the distance data to known grouping, objects (for example, white lines and vehicles in the vehicle surroundings) are recognized in the taken image.
In this distance measuring technique, it is important to verify edge states of stereo-matched pixel blocks in order to enhance reliability of parallaxes calculated for the pixel blocks. One verification method is to evaluate the amount of change in brightness in the lateral direction (horizontal direction) of the pixel block. In this verification method, for example, the amount of change in brightness between pairs of pixels adjacent in the horizontal direction in a certain matched pixel block is compared with a predetermined brightness determination threshold value, and the number of pairs of pixels, in which an amount of change in brightness is more than or equal to the threshold value, is counted. Parallaxes of pixel blocks, in which the counted number is more than or equal to a predetermined number, are output as distance data, and parallaxes of pixel blocks, in which the counted number is less than predetermined number, are removed because of their low reliability.
Japanese Unexamined Patent Application Publication No. 2001-67484 discloses a vehicle surroundings monitoring apparatus in which the number of distance data that varies according to a surrounding environment of the vehicle is optimized. In the vehicle surrounding monitoring apparatus disclosed in this publication, a brightness determination threshold value is adjusted in accordance with the number of distance data that is calculated by comparing the amounts of change in brightness between the adjacent pixels in pixel blocks with the brightness determination threshold value so as to specify pixel blocks in which the amount of change in brightness is large.
Unfortunately, in a case in which the brightness determination threshold value is uniformly increased and decreased on the basis of the number of distance data, as in the above publication, for example, when the brightness determination threshold value is adjusted to a low value, a lot of erroneous distance data based on noise are calculated. In this case, truly necessary distance data, such as distance data on a remote vehicle, may be buried in erroneous distance data. Therefore, from this standpoint, it is preferable that the brightness determination threshold value be kept somewhat high.
On the other hand, in a case in which the brightness determination threshold value is set to be too high, for example, when a flat and patternless wall extends just in front of a subject vehicle in which the apparatus is mounted, it is difficult to calculate a trivial characteristic on the wall as distance data. In this case, accurate recognition of the wall may be impossible.